KR20090017545A - Method for gas phase oxidation using a moderator layer - Google Patents

Abstract

Method for gas phase oxidation, in which a gaseous flow which comprises at least one aromatic hydrocarbon and molecular oxygen is directed through one or more catalyst layers, wherein a moderator layer is arranged between two catalyst layers arranged one behind the other in the flow direction of the gaseous flow, wherein the moderator layer is catalytically less active than the catalysts adjoining upstream and downstream or is catalytically inactive. The desired oxidation products are obtained in high yield for longer periods.

Description

Gas phase oxidation method using mitigating layer {METHOD FOR GAS PHASE OXIDATION USING A MODERATOR LAYER}

The present invention relates to a gas phase oxidation process in which a gas stream comprising aromatic hydrocarbons and oxygen molecules passes through two or more catalyst beds.

Various carboxylic acids and / or carboxylic anhydrides are produced industrially by gas phase oxidation of aromatic hydrocarbons such as benzene, xylene, naphthalene, toluene or durene in a fixed bed reactor. In this way, it is possible to obtain, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride. Generally, a mixture of oxygenous gas and starting material to be oxidized is passed through a tube in which a catalyst bed is present. For temperature control, the tube is surrounded by a heat transport medium, for example a salt melt.

Even if excess heat of reaction is removed by the heat transport medium, a partial temperature peak (hot spot) with a higher temperature than the rest of the catalyst bed can be formed in the catalyst bed. These hot spots can lead to side reactions, for example, the overall combustion of the starting material, or the formation of undesirable by-products (which can only be removed from the reaction product only if very difficult).

In addition, the catalyst can be irreversibly damaged from certain hot spot temperatures. Once the process begins, the loading of the gas stream with the hydrocarbons to be oxidized must initially be kept very low and can only be increased slowly. The final production step is often achieved after a few weeks.

In order to weaken these hot spots, various methods have been taken. In particular, as described in DE-A 40 13 051, a catalyst arrangement of different active layers by layers in a catalyst bed, in which the low activity catalyst is generally located towards the gas inlet and the high activity catalyst toward the gas outlet. The transition has been made.

DE 198 23 262 describes a process for the preparation of phthalic anhydride with three or more coated catalysts in which one layer is arranged on top of another, wherein the catalytic activity is passed from layer to layer from the gas inlet side to the gas outlet side. Increases.

EP-A 1 063 222 describes a process for the preparation of phthalic anhydride carried out in one or more fixed bed reactors. The catalyst bed in the reactor has three or more separate catalyst beds in series in the reactor. After passing through the first catalyst bed, 30 to 70% by weight of o-xylene, naphthalene or a mixture of the two is converted under the reaction conditions. After passing through the second layer, at least 70% by weight is converted.

EP 326 536 describes a process for preparing maleic anhydride by gas phase oxidation of a suitable nonaromatic hydrocarbon via a phosphorus-vanadium mixed oxide catalyst. Mixing the catalyst with an inert solid material lowers the yield reduction per month in long process runs.

WO 2005/063673 describes a process for producing unsaturated aldehydes and / or carboxylic acids by partial oxidation through a fixed catalyst bed, where a layer of inert material is inserted at the point of the reaction zone where the hot spot is expected. The catalysts arranged downstream and upstream of the inert material layer are identical.

The activity of the catalyst or catalyst system used for gas phase oxidation decreases with increasing operating time. A higher proportion of unconverted hydrocarbons or partially oxidized intermediates enters the downstream catalytic bed zone. The reaction gradually moves toward the reactor outlet and the hot spot moves downstream. Catalyst deactivation can be counteracted to a limited extent by increasing the temperature of the heat transport medium. In a multilayer catalyst system, the increase in temperature and / or the movement of hot spots in the heat transport medium results in a rise in temperature such that the gas mixture enters the downstream catalyst bed. Downstream catalyst beds generally have higher activity but lower selectivity, which increases undesirable peroxidation and other side reactions. Overall, product yield and selectivity decrease with process time.

It is an object of the present invention to provide a process by which a preferred oxidation product can be obtained in high yield over a long period of time.

The object is achieved by gas phase oxidation in which a gas stream comprising at least one aromatic hydrocarbon and oxygen molecules is passed through at least two catalyst beds arranged continuously in the flow direction of the gas stream, the activity of the catalysts in adjacent catalyst beds being different and relaxed The layers are arranged between two catalyst beds arranged in series in the flow direction of the gas stream, the relief layer being catalytically less active or catalytically inert than the catalyst adjacent upstream or downstream, and the downstream catalyst layer of the relief layer relaxes. It has greater activity than the catalyst layer upstream of the bed.

As used herein, a catalyst bed refers to a catalyst bed having essentially uniform activity, i.e., having a composition of essentially uniform active composition, active composition fraction and packing density (ignoring unavoidable variations in the filling of the reactor). do. The continuous catalyst bed is therefore different from the activity of existing catalysts. Those skilled in the art will be familiar with various methods of controlling catalyst activity as described below. Within the catalyst bed, the catalyst particles are not diluted by the particles of inert material.

The activity of the catalyst or catalyst bed is tested under the same conditions (particularly with respect to catalyst volume, volume based apparent velocity (gas hourly space velocity, GHSV) and air flow rate, temperature of the heat transport medium, hydrocarbon loading of the gas stream). It is understood to mean the conversion measured within the installation. The higher the conversion of the catalyst or catalyst bed, the higher its activity. This method is particularly suitable for activity comparison and for measuring relative catalytic activity.

The present invention contemplates one or more mitigating beds arranged between two catalyst beds, with the gas stream passing through the mitigating bed before leaving the upstream catalyst bed and entering the downstream catalyst bed.

The relief layer is suitably composed of a bed of particulate material. For easy reactor filling and uniform pressure drop, the particulate material has appropriate dimensions similar to catalyst particles.

The mitigating layer can be catalytically inert. In this case, it consists of an inert material which is also used as a catalyst support, for example. Suitable support materials include, for example, quartz (SiO ₂ ), porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al ₂ O ₃ ), aluminum silicate, steatite (magnesium silicate), zirconium silicate, cerium silicate Or mixtures of such support materials. The softening layer may also include loop-formed knits, loop-drawn knits or fabrics composed of metal wires or fibers.

The mitigating layer may also have catalytic activity. In this case, the mitigating layer is catalytically less active than both the downstream catalyst bed and the upstream catalyst bed. This may be achieved by high content of inactivating additives, low active composition fractions, dilution of the catalyst with inert materials and / or other methods familiar to those skilled in the art. The contribution of the catalytically active mitigating layer to the total conversion in the reactor is preferably small, for example less than 10%, in particular less than 5% of the total conversion. Contribution of the mitigating layer refers to the amount of aromatic hydrocarbon converted in the mitigating layer relative to the total amount of aromatic hydrocarbon converted in the reactor.

The relief layer is arranged between two successive catalyst beds, and the upstream catalyst bed and the downstream catalyst bed have different activities. In this case, the downstream catalyst bed of the relief layer has greater activity than the upstream catalyst bed of the relief layer. The volume ratio of the relief layer to the volume of the upstream catalyst layer of the relief layer is preferably between 0.05 and 0.35, in particular between 0.1 and 0.25.

The volume ratio of the relief layer to the total volume of catalyst and relief layer is generally 40% or less.

In a preferred embodiment, the gas stream passes through at least three catalyst beds, for example three, four or five catalyst beds, arranged continuously in the flow direction of the gas stream. In an embodiment with three catalyst layers, the relief layer is preferably arranged between the second and third catalyst layers. In an embodiment with four catalyst layers, the mitigating layer is preferably arranged between the second and third catalyst layers and / or between the third and fourth catalyst layers.

If multiple catalyst beds are used, various arrangements of activity grades are possible. In a preferred embodiment, the catalytic activity in the flow direction of the gas stream increases gradually from the layer closest to the gas inlet to the layer closest to the gas outlet, from one catalyst bed to the next. The relief layer is preferably arranged between the second catalyst layer and the last catalyst layer at the end, at least in the flow direction.

In another preferred embodiment, the gas stream passes through at least three catalyst beds arranged in series in the flow direction of the gas stream; In this case, the activity of the catalyst increases from one catalyst bed to the next, depending on the order of three or more catalyst beds in the flow direction of the gas stream. Thus, it is possible to provide a relatively short, highly active catalyst layer as the most upstream catalyst layer, followed by a catalyst layer with lower activity downstream, followed by an additional layer with an activity increasing step. have.

The mitigating layer (s) provided in accordance with the present invention result in cooling of the gas stream before entering the catalyst bed arranged downstream of the mitigating layer. Cooling is evident from the temperature profile, ie the temperature plot as a function of catalyst bed length. The temperature profile is easily measurable, for example, by thermoelements arranged in the heat tubes at different heights equidistantly or with thermocouples which are height adjustable.

The temperature at the downstream boundary of the relief layer is preferably at least 2K, in particular at least 4K, lower than the temperature profile extrapolated from the catalyst upstream of the relief layer. The extrapolated temperature profile can be calculated by the following equation.

(δTx)

Where T is the slope of the temperature profile at the boundary of the upstream catalyst from the relief layer and x is the length of the relief layer in the flow direction. The slope of the temperature profile can be measured by taking the tangent to the temperature profile at the boundary of the upstream catalyst of the relaxation layer.

The temperature drop by the relaxation layer is preferably at least 4K, in particular at least 6K and more preferably at least 8K. The temperature drop herein means the difference in temperature at the upstream and downstream boundaries of the relaxation layer, measurable from the temperature profile.

If at least the temperatures of the mitigating layer and catalyst layers adjacent upstream and downstream thereof are controllable together, the reaction tubes in the mitigating layer and the catalyst layer zones adjacent upstream and downstream are surrounded by a single heat transport medium, for example a salt bath. This is desirable. Thus, the method is particularly suitable for reactors with only one heat transport medium circuit.

The process according to the invention is carried out by the process of the anti-corrosive C ₆ -to C ₁₀ -hydrocarbons, for example benzene, toluene, naphthalene or durene (1,2,4,5-tettamethylbenzene) Particularly suitable for gas phase oxidation with acid anhydrides such as maleic anhydride, phthalic anhydride, benzoic acid and / or pyromellitic dianhydride. The method is particularly suitable for preparing phthalic anhydride from o-xylene and / or naphthalene.

The catalytically active composition of all catalysts preferably comprises at least vanadium oxide and titanium dioxide. Measurement of activity control of vanadium oxide and titanium dioxide based gas phase oxidation catalysts is essentially known to those skilled in the art.

For example, a catalytically active composition can include a compound as a promoter that affects the activity and selectivity of the catalyst.

Examples of factors affecting activity and selectivity include alkali metal compounds, in particular cesium oxide, lithium oxide, potassium oxide, sodium oxide and rubidium oxide, and phosphorus or sulfur compounds.

Other means of activity control include changing the proportion of the active composition or the V ₂ O ₅ content in the total weight of the catalyst, with the higher active composition or the V ₂ O ₅ content causing higher activity and vice versa.

Catalysts used in the process according to the invention are generally coated catalysts in which the catalytically active composition is applied in the form of a coating to an inert support. The layer thickness of the catalytically active composition is generally 0.02 to 0.2 mm 3, preferably 0.05 to 0.1 mm mm. The catalyst generally has a layer of active composition applied in the form of a coating to an inert support.

Inert support materials used include aromatic hydrocarbons with aldehydes, carboxylic acids and / or carboxylic anhydrides such as quartz (SiO ₂ ), porcelains, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al ₂ O ₃ ), aluminum silicate, steatite (magnesium silicate), zirconium silicate, cerium silicate, or virtually any prior art support material, such as used advantageously in the preparation of coated catalysts for oxidizing to mixtures of such support materials. . The support material is generally nonporous. Advantageous support materials which can be emphasized are especially steatite and silicon carbide. The form of the support material is generally not critical to the precatalyst and coated catalyst of the invention. For example, catalyst supports in the form of spheres, rings, tablets, helices, tubes, extrudates or cracks can be used. The dimensions of this catalyst support correspond to the catalyst support generally used in the preparation of coated catalysts for the gas phase partial oxidation of aromatic hydrocarbons. Preference is given to using spherical soapstones having a diameter of 3 to 6 mm, or a ring form having an outer diameter of 5 to 9 mm and a length of 4 to 7 mm.

The individual layers of the coated catalyst can be applied by essentially any method known, for example by spray application of a solution or dispersion in a coating drum, or by coating with a solution or dispersion in a fluidized bed. Organic binders, preferably copolymers of vinyl acetate / vinyl laurate, vinyl acetate / acrylate, styrene / acrylate, vinyl acetate / maleate, vinyl acetate / ethylene and hydroxyethylcellulose, advantageously of aqueous dispersion In form, it is possible to add to the catalytically active composition, with an amount of binder of 3 to 20% by weight relative to the solids content of the active composition component solution being advantageously used. If the catalytically active composition is applied to the support without an organic binder, a coating temperature above 150 ° C. is advantageous. When the above-mentioned binder is added, usable coating temperatures depend on the binder used and are 50 to 450 ° C. The applied binder is burned within a short time after introduction of the catalyst and start up of the reactor. Binder addition has the additional advantage that the active composition will be sufficiently adhered to the support to facilitate the transfer and introduction of the catalyst.

In a preferred embodiment of the process according to the invention with three catalyst layers, the catalyst has the following composition (the first layer is the layer arranged most upstream in the flow direction of the gas stream):

First floor:

7 to 10% by weight of active composition, based on the total catalyst

6-11 wt% vanadium pentoxide,

1.2 to 3 weight percent antimony trioxide,

0.1 to 1% by weight of alkali (calculated as alkali metal), in particular cesium oxide, and

As the remainder to 100% by weight, titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 5 to 30 m ² / g, is included.

Second layer:

7-12% by weight of active composition, based on the total catalyst

5 to 13 weight percent vanadium pentoxide,

0-3% by weight of antimony trioxide,

0 to 0.4% by weight of alkali (calculated as alkali metal), in particular cesium oxide,

0 to 0.4 weight percent phosphorus pentoxide (calculated as P), and

As remainder to 100% by weight, titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 10 to 40 m ² / g is included.

3rd layer:

8-12% by weight of active composition, based on the total catalyst

5-30% by weight of vanadium pentoxide,

0-3% by weight of antimony trioxide,

0 to 0.3% by weight of alkali (calculated as alkali metal), in particular cesium oxide,

0.05 to 0.4 weight percent phosphorus pentoxide (calculated as P), and

As the remainder to 100% by weight, titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 15 to 50 m ² / g is included.

The volume ratio occupied by the first layer, the second layer and the third layer is preferably 100 kPa to 200: 40 to 100: 40 to 100.

In a preferred embodiment of the process according to the invention with four catalyst layers, the catalyst has the following composition (the first layer is the layer arranged most upstream in the flow direction of the gas stream):

First floor:

7 to 10% by weight of active composition, based on the total catalyst

6-11 wt% vanadium pentoxide,

1.2 to 3 weight percent antimony trioxide,

0.1 to 1% by weight of alkali (calculated as alkali metal), in particular cesium oxide, and

As the remainder to 100% by weight, titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 5 to 30 m ² / g is included.

Second layer:

7 to 10% by weight of active composition, based on the total catalyst

4-15 weight percent vanadium pentoxide,

0-3% by weight of antimony trioxide,

0.1 to 1% by weight of alkali (calculated as alkali metal), in particular cesium oxide,

0 to 0.4 weight percent phosphorus pentoxide (calculated as P), and

As the remainder to 100% by weight, titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 10 to 35 m ² / g is included.

3rd layer:

7 to 10% by weight of active composition, based on the total catalyst

5 to 13 weight percent vanadium pentoxide,

0-3% by weight of antimony trioxide,

0 to 0.4% by weight of alkali (calculated as alkali metal), in particular cesium oxide,

0 to 0.4 weight percent phosphorus pentoxide (calculated as P), and

As the remainder to 100% by weight, it comprises titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 15 to 40 m ² / g.

4th layer:

8-12% by weight of active composition, based on the total catalyst

10-30 wt% vanadium pentoxide,

0-3% by weight of antimony trioxide,

0.05 to 0.4 weight percent phosphorus pentoxide (calculated as P), and

As the remainder to 100% by weight, titanium dioxide, preferably anatase modified titanium dioxide having a BET surface area of 15 to 50 m ² / g is included.

The volume ratio occupied by the first layer, the second layer, the third layer and the fourth layer is preferably 80 kPa to 160: 30 to 100: 30 to 100: 30 to 100.

If desired, downstream termination reactors as described, for example, in DE-A 198 07 018 or DE-A 20 05 969 A may also be provided for the preparation of phthalic anhydride. The catalyst used is preferably a more active catalyst compared to the catalyst of the last layer.

The catalyst is generally charged to a reaction tube which is thermally stabilized externally to the reaction temperature, for example by a heat transport medium, for example a salt melt. The gas stream passes through the catalyst bed, generally at a temperature of 300 to 450 ° C., preferably 320 to 420 ° C. and more preferably 340 to 400 ° C., and generally 0.1 to 2.5 bar, preferably 0.3 to 1.5 At elevated pressures of bar, they are generally prepared at an apparent speed of 750 to 5000 h ⁻¹ .

The reaction gas supplied to the catalyst is generally a gas containing aromatic hydrocarbons to be oxidized, oxygen molecules and separately from oxygen and suitable reaction modifiers and / or diluents such as water vapor, carbon dioxide and / or nitrogen. Obtained by mixing of the oxygen-containing gas is generally from 1 to 100 mol%, preferably from 2 to 50 mol% and more preferably from 10 to 30 mol% of oxygen, from 0 to 30 mol%, preferably May comprise 0 to 10 mol% water vapor and 0 to 50 mol%, preferably 0 to 1 mol% carbon dioxide, other nitrogen. To obtain the reaction gas, the oxygen molecule containing gas is generally charged at 30 g to 150 g per gas of 1 m ³ (STP) of the hydrocarbon to be oxidized, preferably 60 g to 120 g per 1 m ³ (STP). .

It is also possible for the catalyst bed of two or more zones, preferably two zones, present in the reaction tube to be thermally stabilized at different reaction temperatures, for example a reactor with a separate salt bath can be used. Alternatively, gas phase oxidation may be carried out at one reaction temperature without dividing the temperature zone.

It will be described in detail by the accompanying drawings and the following examples.

Figure 1 shows the temperature profile of a two-layer catalyst system in gas phase oxidation of o-xylene to phthalic anhydride (no relaxation layer).

FIG. 2 shows the temperature profile of a two-layer catalyst system in gas phase oxidation of o-xylene to phthalic anhydride using a catalytically inert relaxation layer between the first and second catalyst layers.

FIG. 3 shows the temperature profile of the fourth layer catalyst system in gas phase oxidation of o-xylene to phthalic anhydride (no relaxation layer).

FIG. 4 shows the temperature profile of a four-layer catalyst system in gas phase oxidation of o-xylene to phthalic anhydride using a catalytically inert relaxation layer between a second catalyst layer and a third catalyst layer.

Comparative Example 1

A commercial two bed catalyst system consisting of two catalysts of different activity was used. The upstream catalyst with lower activity is called the selective catalyst and the downstream catalyst is called the active catalyst. The active compositions of both catalysts include titanium dioxide and vanadium oxide. The active composition was applied to the soapstone support.

From bottom to top, 130 cm of active catalyst and 170 cm of selective catalyst were introduced into the reactor consisting of an iron tube with an average width of 25 mm and a length of 360 cm. For temperature control, the iron tube was surrounded by a salt melt at 350 ° C. 1 m ³ of air to load the o- xylene 80 g of 98.5% strength by weight of sugar (STP), passed through a tube to the bottom of the air in the 4 m ³ (STP) per hour per tube above.

The temperature profile of this catalyst system after 18 months of processing time is shown in FIG. 1. It can be seen that the gas stream enters the active catalyst bed having a temperature of about 385 ° C. The yield after this process time was 113.5 kg phthalic anhydride per 100 kg of o-xylene (based on pure o-xylene).

Example 2

Comparative Example 1 was repeated except that the phase length of the catalyst system was changed to match the relaxation layer. The relief layer consists of a soapstone ring (outer diameter 7 mm, height 7 mm, inner diameter 4 mm). 125 촉매 cm active catalyst, 15 cm relaxation layer and 170 cm selective catalyst were continuously introduced into the tube.

The temperature profile of this catalyst system after 18 months of processing time at a salt bath temperature of 350 ° C. is shown in FIG. 2. It can be seen that the gas stream enters the bed of the active catalyst having a temperature of about 368 ° C., ie, about 17 ° C. lower than the comparative example. The yield after this process time was 114.3 kg of phthalic anhydride per 100 kg of o-xylene (based on pure o-xylene).

Comparative Example 3

The following catalysts 1 to 4 were used. The catalyst comprises the applied active composition and shaped catholyte (magnesium silicate) in cyclic form (7 × 7 × 4 mm, ED × L × ID).

Catalyst 1:

Active composition content: 8.0% of the total weight of the catalyst.

Active composition (after 4 hours calcination at 400 ° C.): 7.1 wt.% V ₂ O ₅ , 1.8 wt.% Sb ₂ O ₃ , 0.36 wt.% Cs.

Catalyst 2:

Active composition content: 8.0% of the total weight of the catalyst.

Active composition (after calcination at 400 ° C. for 4 hours): 7.1 wt.% V ₂ O ₅ , 2.4 wt.% Sb ₂ O ₃ , 0.26 wt.% Cs.

Catalyst 3:

Active composition content: 8.0% of the total weight of the catalyst.

Active composition (after calcination at 400 ° C. for 4 hours): 7.1 wt% V ₂ O ₅ , 2.4 wt% Sb ₂ O ₃ , 0.10 wt% Cs.

Catalyst 4:

Active composition content: 8.0% of the total weight of the catalyst.

Active composition (after calcination at 400 ° C. for 4 hours): 20.0 wt.% V ₂ O ₅ , 0.38 wt.% P.

The reactor used was a reactor consisting of an iron tube with an internal width of 25 mm and a length of 360 cm. The iron tube was surrounded by salt melt for temperature control. Starting with catalyst 4, the catalysts were introduced to have the following phase length distribution: 130/50/70/70 cm 3 (catalyst 1 / catalyst 2 / catalyst 3 / catalyst 4).

To operate the plant, the air / o-xylene mixture was allowed to pass through the main reactor from top to bottom at a salt bath temperature of about 350 ° C.

After the normal run time, the temperature profile specified in FIG. After an extended operating time, a temperature profile characterized by a square representation was obtained. It can be seen that the hot spot profile moves from the reactor inlet to the reactor outlet as the operating time increases. Immediately after operation, the entrance temperature into the third catalyst bed was about 400 ° C., but after an extended operating time it was 440 ° C. This lowers the selectivity and lowers the total yield.

Example 4:

Comparative Example 3, except that a layer of catalytically inert soapstone ring (outer diameter 7 mm, height 7 mm, inner diameter 4 mm) was introduced as a mitigating layer between the layer of catalyst 2 and the layer of catalyst 3 Was repeated. The following phase length distributions were used: 130/70/10/55/55 cm 3 (catalyst 1 / catalyst 2 / relaxation layer / catalyst 3 / catalyst 4).

After the normal run time, the temperature profile specified in FIG. After an extended operating time, a temperature profile characterized by a square representation was obtained. Immediately after operation, the cooling effect of the mitigating layer is relatively mild. Once the hot spot profile moves with increasing uptime, a significant cooling effect of the mitigating layer is observed. As a result, the entry temperature into the third catalyst layer is lowered. This promotes reaction selectivity in the third catalyst bed.

Claims (11)

A gas phase oxidation process in which a gas stream comprising one or more aromatic hydrocarbons and oxygen molecules passes through two or more catalyst beds arranged in series in the flow direction of the gas stream, wherein the catalytic activity in adjacent catalyst beds is different from each other, and the relaxation layer is a gas stream. Arranged between two catalyst layers arranged in series in the flow direction of the buffer layer, wherein the buffer layer is catalytically less active or catalytically inert than the catalyst adjacent upstream and downstream, and the catalyst layer downstream of the buffer layer is upstream of the buffer layer. And a greater activity than the catalyst bed. The method of claim 1, wherein the gas stream passes through at least three catalyst beds arranged in series in the flow direction of the gas stream. The process of claim 1, wherein the catalytic activity in the flow direction of the gas stream continues to increase from one catalyst bed to the next catalyst bed, from the catalyst bed closest to the gas inlet to the catalyst bed closest to the gas outlet. The gas stream of claim 2, wherein the gas stream passes through at least three catalyst beds arranged continuously in the flow direction of the gas stream, and catalyst activity increases from one catalyst bed to the next catalyst bed in the order of the at least three catalyst beds in the flow direction of the gas stream. How to do. The process according to claim 3 or 4, wherein the mitigating layer is arranged at least upstream of the most active catalyst layer and / or the second active catalyst layer. The process according to any one of claims 1 to 5, wherein the ratio of the volume of the relief layer to the volume of the upstream catalyst layer of the relief layer is from 0.02 to 0.35. The temperature of any one of claims 1 to 6, wherein the temperature at the downstream boundary of the relief layer is at least 2K lower than (δTx), and δT is the temperature profile at the boundary of the catalyst upstream of the relief layer from the relief layer. The slope and x is the length of the relaxation layer in the flow direction. The method according to any one of claims 1 to 7, wherein the temperature drop by the relaxation layer is at least 4K. The process according to claim 1, wherein the temperatures of at least the relief layer and the catalyst layer adjacent upstream and downstream thereof can be controlled together. 10. The method of any one of claims 1-9, wherein the catalytically active composition of all catalysts comprises at least vanadium oxide and titanium dioxide. The process according to claim 1, wherein the hydrocarbon used is o-xylene and / or naphthalene and phthalic anhydride is prepared.

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